Pneumatic actuators are used in different types of applications, and serve to actuate or move plungers, shafts and the likes in different types of devices, such as valves.
An actuator can include a normally closed piston, a normally open piston, or both within the same assembly. An actuator including a normally closed piston is typically provided with a biasing element, such as a spring or Belleville washers, which biases the piston in a closed position. In order to move the piston from the closed to the open position, gas is injected within the actuator, counteracting the biasing action of the spring and allowing the piston to move towards the open position. Similarly, normally opened pistons may be biased in their open position and moved to the closed position through the effect of gas pressure thereon.
In order to isolate and seal the chamber in which the gas in injected, dynamic seals are generally used. An example of valve construction based on this principle is shown in FIG. 1 (PRIOR ART). In this example, the normally closed and normally open pistons 12, 14 of an actuating mechanism 10 are both provided with polymer, silicone or rubber O-rings 16 along their outer periphery. When the pistons move up and down within the valve, the O-rings 16 slide and rub against the inner wall of the cylindrical body 18 of the valve.
When actuators are used in high-temperature applications, sometimes as high as 350° C., the dynamic seals tend to become sticky and brittle over time. This increases the friction between the seal and the inner wall of the actuating system, impairing movement of the pistons. Lubricants can be used to minimize this friction, but their use generates other problems: grease-based lubricants, such as Krytox (trademark) or Torr-lube (trademark) eventually dry down, increasing even more the friction and making it harder for the pistons to move. The use of dry lubricants is usually not considered an alternative, as they are eventually blown away by the actuating gas used. Similar difficulties are also encountered in solenoid valves in which the pistons are actuated using an electrical current.
The degradation of the dynamic seals does not only occur in high temperature applications. The problem is also present in cryogenic applications, where valves are used in very low temperature conditions. In this case, the humidity present in the actuating gas tends to crystallize at the interface of the pistons and the inner wall of the actuating system, impairing the movement of the pistons.
There is thus a need for an actuator which can help reduce or eliminate friction between moving parts. For some applications, it would also be desirable for this actuator to impart a higher force to the shaft or plunger it actuates, while keeping the actuating pressure low. For some applications, the mechanism would also need to have a long operating life and be able to move the shaft or plunger at high speed.
Many solutions have been proposed in order to solve this issue. U.S. Pat. No. 5,755,428 shows a valve having a metal-to-metal dynamic seating. While the described type of actuator may be appropriate for ambient or relatively medium temperature, it is not readily usable in high temperature or cryogenic applications. There are 6 O-rings involved in this design. High friction and embrittlement of those O-rings is likely to result if used at cryogenic or at high temperature, like 350° C.
Also known in the art are U.S. Pat. Nos. 5,131,627, 5,215,286, 5,653,419, 6,508,453, 7,159,839 and U.S. 20100090151. Some of these solutions use cam mechanisms, gear-cam and pinion mechanisms, or multiple pistons in order to increase the effective total weight or force to the actuated shaft or rod. These solutions require the use of several small hard-to-machine parts which render the mechanism complex and difficult to assemble or maintain, in addition to increasing the manufacturing costs.
In light of the above, there is a need for an improved pneumatic actuator.